The fatigue behavior of short glass fiber-reinforced styrene-maleic anhydride (S/MA) is investigated in this study. Six material configurations were manufactured into tensile bars by injection molding and tested in tension-tension (R = 0.1) fatigue to evaluate the ways the interface controls the change in the mechanical properties of the composite. These materials included unreinforced S/MA as a reference material and composites incorporating fiber reinforcement with different average diameters, two volume fractions, the presence or absence of an interfacial silane coupling agent, and fibers prepared with an acrylonitrile/butadiene latex coating. These materials have been tested under both “wet” (immersed in water at 25°C) and “dry” (50% relative humidity at 25°C) conditions. Three indicators were monitored during fatigue to evaluate the damage in the experimental materials: the degradation of the secant and fatigue moduli, the hysteresis energy per cycle, and the concurrent acoustic emissions produced during fatigue. This information is presented with S-N curves to show the relationship between these indicators and the fatigue lives of the materials.

The fatigue performance (life curves and cyclic stress-strain curves) was directly related to the fiber-matrix interface. Since water weakens the interface, especially in the composites without a silane coupling agent, composites tested in the aqueous environment exhibited shorter fatigue lives than the specimens tested in the dry environment. Interfacial coatings that included a silane couplant were found to reduce the effects of water on the fatigue lives of the composites. This damage in the composites was reflected in the decrease in the elastic modulus and the increases in the hysteresis energy and acoustic emissions. These evaluation techniques show that fatigue damage accumulates in a three-stage process in the material.